Question

What are the typical situation under which the recycle reactor will be employed?

Answer 1

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Recycle reactors are used when the reaction is autocatalytic, or when it is necessary to maintain nearly isothermal operation of the reactor or to promote a certain selectivity (see Section 5.6.6). They are also used extensively in biochemical operations. To design recycle reactors, one simply follows the procedure developed in this chapter and then adds a little additional bookkeeping. A schematic diagram of the recycle reactor is shown in Text Figure 4-15.       



       The recycled stream is drawn off at point Q and merged with the fresh feed at Point P. We shall define the recycle parameter R as the moles recycled per mole of product removed at point Q.       



     

Two conversions: 
Xsand X0

 Two conversions are usually associated with recycle reactors: the overall conversion, X0, and the conversion per pass, Xs :       




(CD4-88)


(CD4-89)      

The only new twist in calculating reactor volumes or conversions for a recycle reactor is a mole balance at the stream intersections (points P and Q) to properly express the species concentrations as a function of conversion. Consider the gas-phase reaction



occuring in our reactor. Let X be the conversion of A in the reactor per mole of A fed to the reactor. The design equation is

       



       Then:   Design equation:   



      Rate law:   



       with        Stoichiometry:
1. From the definition from the overall conversion, we can defineF A3 and F B3 leaving the system,       




(CD4-90)


(CD4-91)      From the definition for conversion per pass, we can define F A2 and F B3 leaving the reactor,   




(CD4-92)


(CD4-93)      2. From the definition for the recycle parameter, R, we can define F AR and F BR and the total molar flow rate in the recycle stream, F tR       




(CD4-94)



(CD4-95)



(CD4-96)      where   



       3. From the balance on the stream intersections, we have       



(CD4-97)

Relating the molar flow rates in the various streams







(CD4-98)


(CD4-99)


(CD4-100)


(CD4-101)

(CD4-102)

(CD4-103)

(CD4-104)


(CD4-105)

(CD4-106)      The volumetric flow rate in the reactor, , is related to the volumetric flow rate entering the reactor  by       (CD4-107)     where X is the number of moles of A reacted per mole of A entering the reactor, and  is defined by       







(CD4-108)      The molar flow rate of A within the reactor is       



(CD4-109)               (CD4-110)         



(CD4-111)          These equations for concentration are substituted into the rate law, which is in turn substituted into the design equation and integrated. For a first-order reaction in A and in B,       



(CD4-114)    

Recycle reactor volume

 



(CD4-115)      where   



       The relationship between the overall conversion and the conversion per pass can be found by equating F A2 from Equations (CD4-107) and (CD4-106):       



       Then using Equation (CD4-97) and simplifying, we have       



(CD4-116)

Answer 2

Recycle reactors are used when the reaction is autocatalytic, or when it is necessary to maintain nearly isothermal operation of the reactor or to promote a certain selectivity (see Section 5.6.6). They are also used extensively in biochemical operations. To design recycle reactors, one simply follows the procedure developed in this chapter and then adds a little additional bookkeeping. A schematic diagram of the recycle reactor is shown in Text Figure 4-15.

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The recycled stream is drawn off at point Q and merged with the fresh feed at Point P. We shall define the recycle parameter R as the moles recycled per mole of product removed at point Q.

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